Spectrophotometric Determination of Formation Constants of Iron(III) Complexes with Chloride Ion in Dimethyl Sulfoxide

Abstract

The formation constants of FeCl2+ and FeCl2+ have been measured in dimethyl sulfoxide (DMSO) containing Fe(ClO4)3, NaCl, HClO4 and NaClO4. Some spectrophotometric methods giving the formation constants and molar extinction coefficients of the complexes were developed and gave, on the average, logK1=3.62 and logK2=2.10, at 0.02 m HClO4 and an ionic strength of 0.10 m at 25°C. It is rather surprising that these values are only slightly larger than those in N-methylacetamide, which is expected, however, to give much smaller values than DMSO, due to its extraordinarily large dielectric constant. The species formed between Fe(III) and Cl− ion is reasonably supposed to be a true complex bonded directly with each other and not an ion-pair association product. At higher concentrations of Fe(III), formation of polynuclear complexes can occur.